As the speed of computing electronics increases, optical interconnect may replace electrical interconnect between separate chips, or integrated circuit, within modules, or between modules and subsystems of each data center. Among other reasons for considering optical interconnect is that modulated optical signals can be directed over considerable and varied distances at a same power consumption and little crosstalk while electrical interconnect has power demands dependent on interconnect length-dependent capacitance and significant potential for crosstalk.
Use of optical interconnect requires generation, routing, and detection of digitally-modulated optical signals.
Direct modulation of Vertical-Cavity Surface-Emitting Lasers (VCSELs), an approach broadly adopted for off-chip and off-board interconnect at board and rack levels of computing equipment, becomes energetically unfavorable as single channel data rate increases because of the need to switch the voltages and currents such lasers require.
A leading alternative to direct modulation is the use of optical photonic modulators in VCSEL—Photonic Modulator—Waveguide—Detector interconnect configurations.
Photonic modulators can be important optoelectronic devices in photonic datalinks and optical communications where they modulate the intensity and/or the phase of the light, encoding optical signals.
There are generally two types of photonic modulators: (1) electro-optical modulators (EOM), in which the real part of the refractive index is changed (Δn) when a voltage/electric field is applied on the device; (2) electro-absorption modulators (EAMs), in which the imaginary part of the refractive index (Δk)/absorption coefficient (Δα=4πΔk/λ where λ is the wavelength of the light) is changed when a voltage/electric field is applied on the device. Most existing modulators use one of these two mechanisms.
Photonic datalinks and electrical-optical integration in data centers and optical communications requires modulators with low driving voltage, small footprint, low energy consumption, higher extinction ratio (ER, an on/off contrast ratio), and easy optical coupling/integration. EAMs usually have advantages over EOMs in terms of footprint, driving voltage, and power consumption.
Photonic modulators have been demonstrated with modulation rates as high as 500 Gb/s. Surface-incident EAMs, where the incoming light is incident on the surface of the device, are easier to integrate with photonic circuits and datalinks than their waveguide counterparts, therefore are ideal for large scale integration and deployment.